The invention relates to catalysts and catalytic processes for the degradation of perfluorinated compounds and hydrofluorocarbons.
Perfluorinated compounds (PFCs) are used extensively in the manufacture of semiconductor materials, such as dry chemical etching and chamber cleaning processes. PFCs are defined as fully fluorinated compounds made of carbon, nitrogen, or sulfur, or mixtures thereof. Examples of PFCs include nitrogen trifluoride (NF3), tetrafluoromethane (CF4), hexafluoroethane (C2F6), sulfur hexafluoride (SF6), octafluoropropane (C3F8), decafluorobutane (C4F10), and octafluorocyclobutane (c-C4F8).
Hydrofluorocarbons (HFCs) are also used in the manufacture of semiconductor material and are generated as by-products during semiconductor manufacture. HFCs are compounds made of hydrogen, fluorine and carbon. Examples of HFCs include trifluoromethane (CHF3) and 1,1,1,2-tetrafluoroethane (CF3CFH2). The global warming potential of PFC and HFC compounds are estimated to be many times greater than that of carbon dioxide (Langan et al., 1996), resulting in a need for economical technologies for achieving emissions control requirements. Other applications for PFCs and HFCs include uses as polymer blowing agents and as refrigerants.
Catalytic technologies have been and continue to be widely used as an xe2x80x9cend-of-the-pipexe2x80x9d means of controlling industrial emissions. This technology involves passing a contaminated stream over a catalyst in the presence of oxygen and/or water at an elevated temperature to convert the pollutants in the emissions stream to carbon dioxide, water, and mineral acids, should the parent compounds contain halogens. This technology offers many advantages over thermal incineration as a means of controlling emissions. The advantages are connected to the use of the catalyst, which reduces the temperature required to decompose the pollutants by several hundreds of degrees Celsius. These advantages include energy savings (which translates into lower operating costs), lower capital costs, small foot print of resulting abatement unit, a more controllable process, and no generation of thermal NOx.
An important factor in any catalytic abatement strategy is the catalyst itself. The catalytic destruction of PFCs and HFCs results in the formation of highly corrosive fluorine-containing products, such as F2, HF and/or COF2. In order for a catalyst to effectively decompose PFCs and HFCs, the catalyst must be able to maintain its integrity in the resulting highly corrosive environment. Many typical catalytic materials will not maintain their integrity in this reaction environment due to fluorine attack.
Campbell and Rossin (xe2x80x9cCatalytic Oxidation of Perfluorocyclobutene over a Pt/TiO2 Catalyst,xe2x80x9d 14th N. Am. Catal. Soc. Meeting, 1995) suggested the use of a Pt/TiO2 catalyst to destroy perfluorocyclobutene (c-C4F6) at reaction temperatures between 320xc2x0 C. and 410xc2x0 C. The authors reported some loss of reactivity over the duration of the near 100 hour reaction exposure. The authors also note the beneficial effects of water on improving the stability of the catalyst. The authors stated that even at a reaction temperature of 550xc2x0 C., the catalyst did not decompose perfluorocyclobutane (c-C4F8), a PFC used in the manufacture of semiconductor materials. Results presented in this study suggested that perfluoroalkanes are significantly more difficult to destroy than the corresponding perfluoroalkene.
Aluminum oxide, particularly of the high surface area gamma phase, is widely used as a support for catalytically active metals. Aluminum oxide offers a combination of high surface area and excellent thermal stability, being able to maintain its integrity at temperatures of approximately 800xc2x0 C. for short periods of time. Aluminum oxides however, do not fare well as catalyst supports for the destruction of fluorine-containing compounds. For example, Farris et al. (xe2x80x9cDeactivation of a Pt/Al2O3 Catalyst During the Oxidation of Hexafluoropropylene,xe2x80x9d Catal., Today, 501, 1992) report the destruction of hexafluoropropylene over platinum supported on a high surface area aluminum oxide catalyst. It was not reported whether the platinum or the aluminum oxide is responsible for the destruction of hexafluoropropylene. The catalyst could readily destroy hexafluoropropylene at reaction temperatures between 300xc2x0 C. and 400xc2x0 C.; however, severe deactivation of the catalyst was noted. Over the course of the experiment (less than 100 hours), the aluminum oxide was converted to aluminum trifluoride, which resulted in a severe loss of catalytic activity. This transformation of the aluminum oxide to aluminum trifluoride suggests that aluminum oxide will not be able to maintain its integrity in a fluorine environment for an extended period of operation.
Thus, there exists a need for novel and improved catalysts and catalytic processes for the degradation of perfluorinated compounds and hydrofluorocarbon compounds.
Catalyst compositions containing Al2O3 and/or ZrO2 along with one or more enhancers are described. The enhancers may be nickel, cobalt, or sulfate. The catalyst compositions may further contain an oxidation catalyst such as platinum, palladium, rhodium, iridium, silver, nickel, copper, iron, vanadium, or cerium.
The catalyst compositions are particularly useful in catalytic processes for the destruction of perfluorinated compounds and/or hydrofluorocarbons.